I've heard that the Porsche 928 is an exotic car which everyone should own....
On a serious note.... it seems to me that you are interested in a roadster or convertable. Beyond that, what are you seeking?
The S2K, MX-5, and Boxsters are all two-seaters. Is that a requirement for you?
Is RWD and good handling something you are seeking?
Hardtop 'vert or soft-top?
Manual transmission or automatic?
What will be the purpose of the car? Daily driver? Weekend toy car? Occasional autocross? Track rat?
There are many fantastic cars out there for around $20k. If we know more of your requirements, we can narrow it down a bit...
-Z-man.